Contributing editor Benoit Lecavalier recently conducted an extensive interview with Valérie Masson-Delmotte, one of the world's leading climate scientists and the lead coordinating author for Working Group One, the Physical Scientific Basis, in the next IPCC Assessment Report. To read Part I, on climate science, click here.​

Valérie Masson-Delmotte is an internationally renowned climate scientist. She was a lead author for the "paleoclimate" section of the fourth assessment report published by the Intergovernmental Panel on Climate Change (IPCC). She was the lead coordinating author for the "information from paleoclimate archives" section in the IPCC's fifth and most recent assessment report. For the IPCC's upcoming sixth assessment report, she is a lead coordinating author for the entire Working Group One, the Physical Scientific Basis, which oversees all other scientific chapters.

At the recent Second Open Science Conference of the International Partnerships in Ice Core Sciences, in Tasmania, our contributing editor, Benoit Lecavalier, conducted a lengthy interview with Dr. ​Masson-Delmotte. Later this month, we will publish Part II, on  public outreach on the future of the IPCC. 

Benoit Lecavalier (BL): The historical climate record covers a period of direct measurements that inform us about past states of the climate system. How was our knowledge of the climate extended into the past?

Valérie Masson-Delmotte (VMD): First, I would like to say that the instrumental period is in fact sometimes rather short. A few areas go back three centuries, like the meteorological records in England, while on the global scale the records extend back approximately 150 years at best. In some tropical regions, the precipitation records only go back 50 years. While measurements of Antarctic sea ice only begin with satellite measurements starting in ~1979. The instrumental records are very short, and so it is important to complement them with the use of either early instrumental measurements from the historical archives -  for example historical information such as harvest dates, and dates for freezing over lakes and rivers found in Europe, Japan, or China.

Finally, we have to use indirect information from natural archives, where in fact you have continuous recording of environmental parameters related to the mean climate state and variability. Starting basically at the beginning of the 20th century, people have developed methods to make use of all sorts of archives. First from continents, because they were easier to reach, came tree rings and lake sediments, then deep sea sediments, corals, and ice cores.

BL: Can you elaborate on how tree rings bridge the gap between the instrumental and historical period with natural archives, and specifically how they represent a proxy for climate?

VMD: In trees you can measure tree ring widths or density. What I do myself is the measuring of the relative abundance of heavy to light molecules, the atoms in some molecules. I also use the cellulose in the tree rings. In fact, the information on climate that you get depends on the tree's location. It is mostly related to what would be the strongest stress factor for tree growth. In cold areas at the tree line limit, it is temperature, mostly summer temperature, while in some places it can be the availability of moisture.

The tree ring signal therefore depends on the location, and so when people are focusing on temperature then they will look for high elevation and latitude tree ring records where temperature is the key control. These tree ring records are then only selected to build a chronology. We stack the tree ring records and obtain tree samples from different ages with overlap periods with younger trees and older trees to correct for age effects related to the aging of the trees.

Then the tree ring record must be calibrated, which is hard work. We must relate the parameters measured in the tree ring to a climate parameter and it implies a lot of statistical analysis to yield a robust methodology. Then a number of trees are agglomerated for a given region to produce a regional signal, to remove dependence on a single tree in the event that say, an insect attack compromised a tree. Ultimately it is about playing with a large number of records and then agglomerating them at the regional or even hemispheric scale. Depending on the place, tree ring records provide climatic information that can go back in time at the scale of one to two thousand years.

BL: If we want to extend the record even farther back in time, we might look at ice cores. What types of measurements are conducted on ice cores and how do those proxies relate to environmental conditions?

VMD: An ice core is basically a record of atmospheric deposition, so you have information on what remains of past snowfall after it has been eroded by winds. Furthermore, you have information on temperature through a number of direct and indirect parameters. The direct information arises when you drill an ice core  - you are left with a hole in the ice and you can measure a temperature profile down this borehole. Any change in surface temperature will be imprinted but filtered by diffusion processes in this temperature profile. Therefore, borehole temperatures are probably the most direct information. When you have summer temperatures above zero degrees you have melt and refreezing and so melt layers form, a crucial piece of information on past temperature from ice core.

Myself, I work on water stable isotopes, again measuring heavy to light water molecules directly in the water from the ice cores. And these are reflecting past changes in moisture transport from ocean and continental evaporation areas towards the central part of the ice sheets. Since the 1960s, we have greatly expanded our physical understanding of how hydrogen and oxygen isotope fractionation relates to past surface temperatures from theoretical models. We can also simulate it in atmospheric models.

BL: We have temperature reconstructions from borehole temperatures, summer melt layers, and water isotopes from ice core studies, we also have additional information from the air trapped in the ice cores. What can be said of the ancient gases trapped in air bubbles, such as CO2?

VMD: The instrumental records on atmospheric CO2 go back to the 1960s. We don’t have high quality CO2 records from Greenland ice cores, but we do have them for Antarctica. The issue is that Greenland is closer to the continents. It receives a lot of organic matter and dust from the nearby continents. There are chemical reactions taking place in the Greenland ice that make the preservation of the atmospheric composition of CO2 problematic. Instead, we rely on the analysis of the air trapped in the firn of Antarctic and in the ice cores. However, Greenland ice core remain very useful for methane and other gases, but not for CO2 unfortunately.

BL: Ice cores yield a great deal of information on past climates. What are some novel inferences that have come out of ice cores in the past several years?

VMD: What I like about ice cores is that we have local information, the amount of accumulation/precipitation, local temperature, then you have original information from the chemical analysis of the ice core, transport of sea salt, transport of dust from the continent, biomass burning from the continents, what we call aerosols, and then we can have more remote information. By looking at water isotopes, we have information on the evaporation conditions of water, not local but at the moisture source. We also have information on climate forcing through the deposition of volcanic aerosols, and through the impact of solar activity on cosmogenic aerosols. This allows us to reconstruct the external drivers of climate change.

Finally, we have global information through the analysis of the atmosphere composition. This helps us place our current changes in a broader context, and it facilitates our understanding of how the global biogeochemical cycles work, from the past methane and CO2 cycle. The new information that arose from the long 800,000 year-old Antarctic ice core, which shed light on glacial-interglacial cycles, demonstrated relationships between atmospheric composition and climate, as well as between ice volume, sea level, and climate. We also have a lot of information on abrupt events of the last glacial periods which are characteristic of spontaneous climate variability, not related to the position of the Earth around the sun, which is the first-order driver of glacial-interglacial changes. These abrupt climate events arise from the internal reorganization of the climate system. We describe these events in more and more detail using data and models but we don’t know exactly the cause.

More recently, a lot of information has arisen that compares the current interglacial with earlier and warmer ones. This helps us understand what happens when the climate is warmer than it is now. This research is not really about analogues but about case studies where you know what is the perturbation to the climate system. Based on this, you can test climate feedbacks under similar ranges of actions or "forcings" [climate influences]. Like what happened when climate in the atmosphere was two to four degrees celsius warmer than today, we have information of the past on that.

We can use this data to test climate models, which is I think very important. Climate models developed for today based on process analyses and physics can be further validated by testing them on case studies for extreme cold and extreme warm conditions, to make sure that the models have the right feedbacks. At the moment, it appears we can start testing the models not only for the mean states but also for transient changes, like glacial-interglacial transitions.

Finally, working on the last 2000 years is very important, it is a challenge for paleoclimatologists because we are looking for a small signal in a lot of noisy data. What is emerging is a detailed picture of regional changes around Antarctica and Greenland, changes in atmospheric and ocean circulation, the response to volcanic forcing, and modes of variability. All of these things will also be at play in the future in addition to being superimposed on the human influence to the system.

​This interview has been lightly edited for readability. Stay tuned for Part II, later this month. ​

For the third episode of the Climate History Podcast, Dr. Dagomar Degroot interviews two leading archaeologists of the medieval and early modern Arctic: Dr. Thomas McGovern of the City University of New York, and Dr. George Hambrecht of the University of Maryland College Park. Since 1972, Professor McGovern has travelled the world for archaeological fieldwork. He has spent much of his time in the far north, especially in Greenland, Iceland, the Faeroes, and Shetland. He is one of the founders of the North Atlantic Biocultural Organization (NABO), and he is associate director of the Human Ecodynamics Research Center at the CUNY Graduate Center. He has an extraordinary record of groundbreaking publications on the history of Arctic peoples, and he has won many awards from the National Science Foundation, the Ford Foundation, the National Geographic Society, and other leading organizations. 

Professor Hambrecht is part of a new generation of interdisciplinary archaeologists of far north. He explores interactions between the political, environmental, and biological dimensions of the transformative early modern period. He is especially interested in historical and zooarchaeology in Iceland, the Subarctic, and the Arctic North Atlantic. He has done extensive fieldwork to reconstruct how Icelandic farmers coped with the onset of the Little Ice Age. He leads teams of interdisciplinary researchers in projects that have recently won support from the National Science Foundation. ​

In this episode, professors Degroot, McGovern, and Hambrecht discuss the perils of doing fieldwork in the Far North; the struggles of the Norse in Greenland and Iceland at the onset of the Little Ice Age; the threat of climate change to the archaeological record of northern peoples; and the possibilities (and challenges) of interdisciplinary approaches to Arctic research. 

To listen to this episode, click here to subscribe to our podcast on iTunes. If you don't have iTunes, you can still listen by clicking here.

Photo credit: Benoit S. Lecavalier (2014).

In the second episode of the Climate History Podcast, Dr. Dagomar Degroot interviews the co-founder and co-administrator of the Climate History Network: Dr. Sam White of Ohio State University. Professor White is one of the most innovative and respected environmental historians of the "Little Ice Age," a period of climatic cooling that, according to some definitions, affected most of the world from the fourteenth through the nineteenth centuries. 

Professor White's first book, The Climate of Rebellion in the Early Modern Ottoman Empire, was published by Cambridge University Press in 2011. In it, Professor White uses interdisciplinary methods and sources to explore how climatic cooling and drying shaped the history of the Ottoman Empire. The book won the Middle East Studies Association Albert Hourani award, the Turkish Studies Association Fuat Köprülü award, and the British-Kuwaiti Friendship Society prize for the best book in Middle East and Turkish studies. Professor White has also written award-winning articles, including one entitled "From Globalized Pigs to Capitalist Pigs: A Study in Animals Cultures and Evolutionary History," which was published in the journal Environmental History in 2011. It won the American Society for Environmental History Leopold-Hildy Prize and the Agricultural History Association Wayne D. Rasmussen Award. More recently, Professor White has received major fellowships to write a book that traces how climate change influenced the early settlement of North America. It is currently under contract with Harvard University Press. 

For our first episode, our founder, Dr. Dagomar Degroot of Georgetown University, interviewed one of the world's best-known and most respected historians: Dr. Geoffrey Parker, of Ohio State University. Professor Parker has authored, edited, or co-edited 37 books and more than 100 articles and book chapters. Recently, he released a groundbreaking new book called Global Crisis, which explores how a cooling climate disrupted societies across the seventeenth-century world. Global Crisis has won awards that include the prestigious Heineken Prize, given by the Royal Netherlands Academy of Arts and Sciences, and the British Academy Medal, given by the British Academy for the Humanities and Social Sciences. We were thrilled to have Professor Parker as the first guest on our podcast.

To listen to our inaugural episode, click here to subscribe to our podcast on iTunes. If you don't have iTunes, you can still listen by clicking here.   

Photo credit: Iceberg off Greenland, by Benoit S. Lecavalier (2014).

Granted, most natural history museums have exhibits dedicated to climate history and global warming. The Smithsonian's National Museum of Natural History in Washington, D. C., for example, features a spectacular exhibit that links climate change to the fate of early hominids (above). The American Museum of Natural History in New York has an alcove just beneath the Hayden Planetarium that describes global warming and explains how scholars reconstruct past climates. But no permanent museum gathers all of this information in one place, in a way that skips over disciplinary boundaries and alerts the public about the importance of action, today.


Miranda Massie and a group of like-minded experts are trying to change all that. Massie is the executive director of the Climate Museum Launch Project. Her initiative seeks to open a climate change museum in New York City, which she hopes will attract some one million visitors per year. We are delighted that she took the time to answer our questions.  

DD: Will your museum explore how humans have evolved with climate change, from prehistory to the present? Or will this be a museum of global warming? 

MM: The main focus will be on solutions to anthropogenic climate change, with concentric circles of context. One of those circles will be devoted to the history of the planet’s climate and its relationship to life, including human life.

DD: To what extent will disaster and decline – keywords in the quest for action on global warming - play a role in your museum? 

MM: While we must be honest about growing risks, this is very much a museum about resilience, adaptation, and mitigation—about our shared ability to escape the danger our shared abilities and proclivities have created.  

DD: How close is the Museum of Climate Change to becoming a reality? What still needs to be done? How can people help? 

MM: Much closer than it was when we started fifteen months ago, with longer left to go. We’ve gotten as far as we have because so many people have offered support and expertise, and we very much welcome more. It is a huge help for people to spread the word through social media and in person. And we gratefully welcome your readers’ further thoughts on how we can, with their engagement and support, get closer to the red ribbon moment. I can be reached at [email protected].

Dagomar Degroot (DD):  In your most recent book, The Hockey Stick and the Climate Wars, you chronicle the campaign of misinformation surrounding a widely cited climate reconstruction you published in 1999. Why does the past matter so much for our understanding of present and future climate change? Were you surprised when your reconstruction thrust you into a debate about contemporary policy? 

Michael Mann (MM): In reality, the “Hockey Stick” (or indeed, the “Hockey Steam” of dozens of reconstructions published over the decade and a half since our original work by other scientists arriving at the same conclusions) has never been the central pillar of evidence for human-caused climate change that the critics make it out to be. There are dozens of independent lines of evidence that human beings are warming the planet and changing the climate through fossil fuel burning and other activities. However, the Hockey Stick demonstrates in a very visual, easily understood way how unprecedented the human-caused warming of the planet is. That made it a threat to vested interests opposed to taking action to curb carbon emissions. And thus did I find myself in the center of the larger campaign to discredit climate change, as I describe in the book. I was surprised at the time that the attacks were so nasty, ad hominem, and factually challenged. Perhaps I should not have been however. I have grown to realize that there is virtually nothing polluting interests won’t stoop to in their efforts to fool the public and policymaker about the threat posed by climate change. 

DD:  In your opinion, is climate change scholarship necessarily political in today’s world? Do climate scholars have a duty to influence policy? 

MM: Unfortunately, it has become political, because vested interests have sought to divide the public on the issue, recognizing that a divided public is all you need to insure inaction. It wasn’t too long ago that there were politicians on “both sides of the aisle” who recognized the need to act on the climate change threat. In my book, I describe how former Republican House Science Committee Chair Sherwood Boehlert (R-NY) defended climate science and scientists when they were under attack by some of his fellow republicans. In this environment, it is essential that scientists and science communicators fight back against the politically-motivated attacks on their science. I expand on this theme in my op-ed in the New York Times earlier this year, entitled “If you see something, say something." I argue that if scientists do not attempt to inform the public discourse over this vital issue, then we all lose out because this creates a void which will readily be filled by those with an agenda of denial and inaction.

Dr. Michael Mann is one of the world's best-known climate change scholars. He is the director of the Earth System Science Centre at Pennsylvania State University, where he pioneers innovative methods for reconstructing past climate change. He is perhaps best known for a thousand-year climate reconstruction he co-authored in 1999, which featured prominently in both the IPCC's Third Assessment Report (2001) and in Al Gore's An Inconvenient Truth. Mann is the author of more than 160 peer-reviewed publications and two books, and he is the founder of the popular climatology blog RealClimate. On Twitter, he is followed by nearly 23,000 people. 

For these reasons, we asked Dr. Mann to give an interview that would launch a new "Interviews" section of HistoricalClimatology.com. We are very grateful that he took the time to answer our questions. 

DD: Recently, Neil deGrasse Tyson has called for more climate scholars to popularize climate issues as he has popularized astronomy. You are perhaps the world’s most famous climate scientist. How would you respond to Tyson? How can climate scholars connect with a sceptical public? 

MM: Well, I doubt I’m the world’s most famous scientist, but I appreciate the thought! I naturally agree with deGrasse Tyson (and I’m very much an admirer of his efforts to communicate science to the public), and indeed this was a primary focus of my recent New York Times op-ed.  The late Stephen Schneider spoke eloquently of the double ethical bind that we face: we must strive to communicate effectively but honestly, even in the face of an opposition that is unfettered by this latter constraint.  We must convey what is known in plainspoken jargon-free language, while acknowledging the real uncertainties that exist. Further, we must explain the implications of those uncertainties, which in many cases imply the possibility of greater, not lesser, risk. Finally, we must not be averse to discussing the policy implications of the science, lest we fail to provide our audience with critical information that can help them make informed choices about their own actions as citizens.

DD: Scholars in more and more disciplines are now considering issues related to climate change. As a scientist, what kind of scholarship in “non-scientific” disciplines do you find most interesting? 

MM: Well, I’m very much a believer in the importance of interdisciplinary research, and climate change is an example of an area of science where interdisciplinary approaches are critical. Obviously, there is physics, chemistry, and math involved--and also biology, since life plays an active role in the climate system (e.g. the carbon cycle). But because climate change impacts so many aspects of life, i.e. human health, water resources, food, land, national security and conflict, and our economy, it is also critical that we interact with scientists in the social sciences, i.e. economics, sociology, political science, and psychology. Beyond this, however, are the ethical dimensions of the problem, i.e. how to balance the interests of the developed world and the developing world, how to balance the costs today with the costs that will be born by future generations. So disciplines of the humanities such as ethics, are critical. And of course, in communicating the science and its implications to the public, we must work with experts in communication and the arts. In my own efforts, both research and outreach, I find myself interacting with individuals in literally every one of the fields and disciplines mentioned above. 

DD: Is there a “pause” in global warming? What does the controversy about a pause reveal both about climate science and its “sceptics?”  

MM: There has been a temporary slowdown in warming, what I refer to as a “faux pause” in my Scientific American article earlier this year. The slowdown is almost certainly fleeting, due to a number of natural factors (La Niña, downturn in solar output, and an increase in volcanic activity) that have temporarily masked some of the warming that otherwise would have taken place. This is likely coming to an end. 2014 is on track to be the warmest year on record.

However, with climate change deniers, this has never been about the science. They’ll typically grab onto any misleading talking point or myth that they think they can invoke to manufacture doubt about human-caused climate change. So we can expect that when 2014 sets a new global temperature record, they’ll move on to other misleading talking points.

DD: Finally:  what are you currently working on?

MM: Lots of different things. I am working on a number of different projects that deal with climate change impacts on human health, and water resources, efforts to isolate the true impacts of natural climate variability in recent decades, and work aimed at understanding the factors behind the natural climate changes of past centuries. And of course, there is teaching, advising, and public outreach and communication. Suffice it to say, I manage to keep busy!